Parametric study on the performance of a direct-expansion geothermal heat pump using carbon dioxide

A numerical model has been developed to analyze the steady state performance of a direct-expansion geothermal heat pump using carbon dioxide in a transcritical cycle. The system consists of a compressor, counter-flow gas cooler/water heater, expansion device and the ground-coupled evaporator coils. The simulation results are found to be in good agreement with the results available in the literature. To analyze the performance characteristics of carbon dioxide in the ground-coupled evaporator, the parametric study focused on evaporator parameters including coil length, number of ground circuits and mean evaporation temperature. Also investigated is the relationship between gas cooler and evaporator sizing. Results indicate that there exists an optimum number of ground circuits which results in higher heating capacity and coefficient of performance. Performance can be further improved by optimizing the mean evaporation temperature with the surrounding soil temperature. With optimization, the system under study could achieve a coefficient of performance of 2.58, representing an 18% improvement compared to the baseline system.